Process for forming a perforated thermoplastic sheet



J. BJORKSTEN Aug. 1, 1961 PROCESS FOR FORMING A PERFORATED THERMOPLASTIC SHEET 2 Sheets-Sheet 1 Filed Nov. 30, 1954- IN V EN TOR. BJORKS TEN BY i ifimw Attorney Aug. 1, 1961 BJQRKSTEN 2,994,108

PROCESS FOR FORMING A PERFORATED THERMOPLASTIC SHEET Filed Nov. 30, 1954 2 Sheets-Sheet 2 IN VEN TOR. JOHA BJOR/(S TEN XQZL/ Altar/r ey United States Patent 2,994,108 PROCESS FOR FORMING A PERFORATED THERMOPLASTIC SHEET Johan Bjorksten, Fitchburg, Wis. (2202 Sul Ross St., Houston 6, Tex.) Filed Nov. 30, 1954, Ser. No. 472,072 Claims. (Cl. 18-475) This invention relates to a method for increasing the strength and controllability of parachutes made from stretched or blown film. This application is a continuation-in-part of my copending application, Serial No. 246,308 filed Sept. 12, 1951 for a method of making parachutes from stretched or blown film.

There are many difficulties involved in employing film in parachute manufacture:

(1) In order to obtain the very high mechanical strength characteristics of nylon, it has been necessary to draw fibers of this material. Cast film has only a small fraction of the tensile strength of the textile fiber.

(2) A parachute, to be controlled, must be porous. Ordinary continuous film, because of its non-porous nature, does not slow the passage of the air, but rather traps it.

' (3) In all types of parachutes the attaching of the shrouds to the canpoy has been a major problem. The strain of the load must be transmitted to the canopy.

An object of this invention is therefore a method of making a substantially non-fibrous parachute having a very high mechanical strength.

Another object of this invention is a method of making a substantially non-fibrous parachute having the required porosity.

Another object of this invention is an improved method of attaching the shrouds to the canopy.

Further objects will become apparent from the drawings and the following detailed description in which it is my intention to set forth the applicability of the invention without thereby limiting its scope to less than that of all equivalents which will be apparent to those skilled in the art.

- FIGURE 1 is a cross-sectional view showing a synthetic polymeric resinous material being expanded inside a forming die.

FIGURE 2 shows the material expanded until it projects out the opening in the die.

FIGURE 3 is a cross-sectional view showing a sheet of the said material held circumferentially and expanded by vapor pressure.

FIGURE 4 is a cross-sectional view with the sheet being expanded between a male and a female die.

FIGURE 5 is a cross-sectional view enlarged to show the material in layers.

In accordance with one embodiment of the invention I blow a synthetic polymeric resinous material into a soap bubble-like shape within a large hollow globe having the shape of a spheroid, that is, in the non-mathematical sense, a shape which at least resembles a sphere. By this procedure I obtain the material in film form and still obtain the elongation and orientation which is necessary for extreme strength.

Prior to the blowing of the bubble a pressure-sensitive tape is applied to the inside of the globe with a water soluble adhesive. The integration of the tape and the synthetic polymeric resinous material will serve two purposes. The tape will limit the propogation of tears in the film in such a manner that the requisite strength characteristics are obtained. The tape further furnishes a strong integral part of the canopy to which the shrouds can be attached.

Referring now to FIGURE 1, the melted plastic composition 1 is fed into a blowing cup 2 through a heated 2,994,108 Patented Aug. 1, 1961 tube 3. The temperature in tube 3 and cup 2 is maintained by means of heating coil 4. When the desired amount of plastic has been delivered to the cup, air is injected from below through the pipe 5. This air causes the plastic to be blown out into a bubble form. as a large bubble 6. This bubble expands until it touches the inside walls of the two hemispheres 7 and 8, which are laid up so as to form a shell surrounding around the bubble. Instead of being hemispheres, the mold parts may instead be any sections of a spheroid.

Inside these hemispheres there is previously laid out a reinforcing pattern made of high strength adhesive tape 9.

This adhesive tape is preferably reinforced with glass fibers or with a high strength textile fiber, such as, for example, Fortisan or nylon. This adhesive tape may hold for example 500 pounds per half inch width. It carries on its outside surface a contact-sensitive watersensitive adhesive and on its inside surface, facing the hollow part of the hemisphere, a pressure-sensitive, sub stantially contact insensitive water resistant adhesive. As the bubble expands toward the point of contacting the inside of the outer shell, the air previously within the globe formed by hemispheres 7 and 8 escape through a multitude of perforations or small holes 10, so that finally the expanding bubble fits snugly under pressure inside the shell.

Air pressure is then increased so that finally the pressure-sensitive adhesive on the tape 9 is activated and the tape structure thus made to adhere firmly to the expanding bubble 6. The pressure may then be still further increased until the bubble material is forced out through the small holes 10, forming small bubbles 11 outside the shell, as shown in FIGURE 2. Small bubbles 11 are then either broken by over-expansion with this pressure or are fused or melted outside the shell so that small holes are formed in the large bubble.

The globe thus held under pressure is then cut through.

the necessary harness has been attached to its tape re-.

inforced edges.

In this manner are obtained parachutes of hemispherical shape reinforced by high strength tape and provided with escape apertures for air so as to permit the proper degree of inflation and motion of the parachute when under influence of gravity and load.

An alternative method is shown in FIGURE 3. A flat sheet of a synthetic polymeric resinous material 6' is secured between two halves of a mold 7 and 8'. Then heated air or steam is forced into the mold through 5' and expands the plastic material into position.

Another alternative can be used to expand the plastic into position. As shown in FIGURE 4 a male die 8" electrically heated by means of heating element 15 and forced upward by plunger 16 is shown in the early stages of forming a parachute canopy by forcing synthetic resin sheet 6" upward into contact with strips 9 and the interior mold 7. Porosity may then be provided by introducing air under pressure between the die 8" and sheet 6" to form bubbles such as bubbles 11, which may be broken or severed as described above.

While in the above description reference has been made particularly to nylon as a material for parachutes, parachutes can be made by the same method from, for example, polyvinyl chloride, which has a tensile strength closely approaching that of nylon when drawn and which 3 has viscosity characteristics rendering it, when properly compounded and stabilized, suitable for these purposes. Also suitable may be polyurethane resins, acrylate resins, and generally any thermoplastic resins in the viscosity and molecular size range and having the requisite strength which make them adapted for the blowing techniques'out lined.

With references to the adhesive tape material discussed, while I prefer to have the contact and pressure sensitive adhesive distributed onthe tapes 9 as indicated above, it would also be possible to use contact sensitive adhesives on both sides of the tapes 9, in which case the tape material would be first applied with a contact sensitive adhesive attaching it to the inside of the hollow hemispheres, and contact sensitive adhesive suitable for holding the film would then be appliedto the tape from the under side of the hemispheres by a brushing or spraying operation.

Tape may also be heat sealed to the filtnafter the film has been blown into contact'with the hemispheres. The film is held in position by means of suction applied through the holes 10 and the tape is heat sealed to the inside of the film .bubble.

Furthermore it is obviously possible to apply a succession of bubbles inside each other before their removal, thus obtaining a laminated structure with its concomitant advantages.

This is illustrated in FIGURE which shows a cross section of a film having three plys, 19, 20, and 21. A small hole 14 is shown and a section of reinforcing tape 9, attached with adhesive 22,

FIGURES 1 through 5 have shownthe bubble as being blown vertically upward into a spheroid formed by hemispheroids disposed substantially vertically above theblowing cup. The process may be carried out with'success in the opposite manner with the bubble being blown downward into hemispheroids which form'a spheroid disposed vertically below the blowing cup.

In connection with previous figures and' examples, air has been described as the gas to be used in blowing-the bubble, when in fact any gas suitable for'use under such conditions can be used. Such a gas would ordinarily be required to be non-poisonous, non-corrosive, relatively inexpensive, chemically stable, non-explosive, and noninflammable, but none of these conditions are absolute requirements and types of gases possessing one or more of these properties might be desirable. For example, an otherwise undesirable gas might be found useful in chemically treating the surface of the film at the sametime that the said film was being blown. Thus, as well as air, the gases which might be used include oxygen, nitrogen, chlorine, helium, and others. Helium in particular is useful because of its low specific gravity which may be utilized to control the shape of the bubble as'it is'being' blown.

4 variation in its details and is not to be restricted to the specific examples given above which are given by way of example and not of limitation and that it is to be restricted only by the claims in which it is my intention to cover all novelty in the invention as broadly as possible in view of the prior art.

Having thus disclosed my invention, I claim:

1. The process of forming an article of the type described comprising providing a sheet of thermoplastic polymer adjacent a perforated rigid member conforming to the shape of the article desired, then forcing portions of said thermoplastic sheet adjacent said perforations through said perforations by creating a fluid pressure differential across said sheet and said rigid member to thereby provide freely expanded unrestricted bubbles of said thermoplastic polymer extending from said perforated member opposite to said sheet, and then destroying said bubbles to thereby provide openings extending entirely through said thermoplastic sheet corresponding to the perforations in said rigid member.

2. Method of claim 1 wherein said bubbles are destroyed by severing.

3. Method of claim 1 where said bubbles are destroyed by overexpansion.

4. Method of claim 1 wherein said bubbles are destroyed by melting.

5. A process of forming a perforated thermoplastic sheet which comprises positioning a heated thermoplastic sheet adjacent a perforated rigid mold member, forcing the portions of said thermoplastic sheet, adjacent the perforations of said mold member, through said perforations by creating a fluid pressure differential across said sheet and said rigid mold member to thereby provide freely expanded unrestricted bubbles of thermoplastic material extending from said perforated rigid mold member opposite to said thermoplastic sheet, destroying said bubbles to thereby provide openings extending entirely through said thermoplastic sheet corresponding to the perforations in said rigid mold member, and removing the perforated thermoplastic sheet from the rigid mold member.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,564 Hobson Oct. 14, 1952 825,977 Neville July 17, 1906 2,082,715 Nadai June 1., 1937 2,286,117 Sidnell June 9, 1942 2,309,107 Giroux Jan. 26, 1943 2,354,916 Hurt Aug. 1,1944

FOREIGN PATENTS 606,235 France June 9, 1926 271,488 Great Britain July 14, 1927 358,011 Great Britain Oct. 1, 1931 

